Energy Control Centers 1.0 Introduction The energy control

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Energy Control Centers 1.0 Introduction The energy control

Transcript Of Energy Control Centers 1.0 Introduction The energy control

Energy Control Centers 1.0 Introduction The energy control center (ECC) has traditionally been the decision-center for the electric transmission and generation interconnected system. The ECC provides the functions necessary for monitoring and coordinating the minute-by-minute physical and economic operation of the power system. In the continental U.S., there are only three interconnected regions: Eastern, Western, and Texas, but there are many balancing areas (also known as control areas), with each balancing area having its own ECC. Figure 1 illustrates the balancing areas within the US.

Fig. 1: U.S. Energy Control Centers [0]
Maintaining integrity and economy of an interconnected power system requires significant coordinated decision-making. So one of the primary functions of the ECC is to monitor and regulate the physical operation of the interconnected grid.
Most areas today have a two-level hierarchy of ECCs with the Independent System Operator (ISO) performing the high-level decision-

making and the transmission owner ECC performing the lower-level decision-making.

A high-level view of the ECC is illustrated in Fig. 2 where we can identify the substation, the remote terminal unit (RTU), a communication link, and the ECC which contains the energy management system (EMS). The EMS provides the capability of converting the data received from the substations to the types of screens observed in Fig. 2.

Remote terminal unit


SCADA Master Station
Energy control center with EMS

Communication link

EMS 1-line diagram

EMS alarm dis2p7 lay

Fig. 2: Basic Components of Energy Control Centers


In these notes we will introduce the basic components and functionalities of the ECC. Note that there is no chapter in your text which provides this information.
2.0 ECC Components The system control function traditionally used in electric utility operation consists of three main integrated subsystems: the energy management system (EMS), the supervisory control and data acquisition (SCADA), and the communications interconnecting the EMS and the SCADA (which is often thought of as part of the SCADA itself). Figure 3a [1] provides a block diagram illustration of these three integrated subsystems and Fig. 3b provides a closer view. The SCADA and communications subsystems are indicated in the dotted ovals at the top left hand corner of the figure. The rest of the figure indicates the EMS. We will describe each one in the following subsections.

Figure 3a: System control subsystems EMS, SCADA, and Communications [1]

Fig. 3b: Figure 3a: System control subsystems EMS, SCADA, and Communications [1]
We distinguish EMS from distribution management systems (DMS). Both utilize their own SCADA, but for different functions. Whereas EMS/SCADA serves the high voltage bulk transmission system from the ECC, the DMS/SCADA serves the low voltage, distribution system from a distribution dispatch

center. We are addressing in these notes the EMS/SCADA.
There are two parts to the term SCADA [2-5]. Supervisory control indicates that the operator, residing in the energy control center (ECC), has the ability to control remote equipment. Data acquisition indicates that information is gathered characterizing the state of the remote equipment and sent to the ECC for monitoring purposes.
The monitoring equipment is normally located in the substations and is consolidated in what is known as the remote terminal unit (RTU). Generally, the RTUs are equipped with microprocessors having memory and logic capability, together with some form of telemetry to provide the communication link to the ECC.

Relays located within the RTU, on command from the ECC, open or close selected control circuits to perform a supervisory action. Such actions may include, for example, opening or closing of a circuit breaker or switch, modifying a transformer tap setting, raising or lowering generator MW output or terminal voltage, switching in or out a shunt capacitor or inductor, and the starting or stopping of a synchronous condenser.
Information gathered by the RTU and communicated to the ECC includes both analog information and status indicators. Analog information includes, for example, frequency, voltages, currents, and real and reactive power flows. Status indicators include alarm signals (over-temperature, low relay battery voltage, illegal entry) and whether switches and circuit breakers are open or closed. Such information is provided to the ECC through a periodic scan of all RTUs. A 2 second scan cycle is typical.

2.2 Communication technologies The form of communication required for SCADA is telemetry. Telemetry is [6] the measurement of a quantity in such a way so as to allow interpretation of that measurement at a distance from the primary detector. The distinctive feature of telemetry is the nature of the translating means, which includes provision for converting the measure into a representative quantity of another kind that can be transmitted conveniently for measurement at a distance. The actual distance is irrelevant.
Telemetry may be analog or digital. In analog telemetry, a voltage, current, or frequency proportional to the quantity being measured is developed and transmitted on a communication channel to the receiving location, where the received signal is applied to a meter calibrated to indicate the quantity being measured, or it is applied directly to a control device such as a ECC computer [2].

Forms of analog telemetry include variable current, pulse-amplitude, pulse-length, and pulse-rate, with the latter two being the most common. In digital telemetry, the quantity being measured is converted to a code in which the sequence of pulses transmitted indicates the quantity. One of the advantages to digital telemetering is the fact that accuracy of data is not lost in transmitting the data from one location to another [2]. Digital telemetry requires analog to digital (A/D) and possible digital to analog (D/A) converters, as illustrated in Fig. 4 [2].

Quantity to be


A/D converter

Telemeter transmitter

Telemeter receiver

Signal circuit (wire line, PLC, microwave, etc.)

D/A converter

Indicating meter
Recording meter

Fig 4: Block Diagram of Telemetering System [2]
The earliest form of signal circuit used for SCADA telemetry consisted of twisted pair wires; although simple and economic for short